This invention relates generally to wireless communications, and more particularly to an improved system and method for wideband for tracking the carrier frequency of a multipathed transmission.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers. Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum. Further, receivers can be designed to take advantage of multipath. These system characteristics encouraged early development of the technology by the military.
Common forms of spread spectrum systems include chirp, frequency hopping, and direct sequence or pseudonoise (PN). The chirp system transmits an impulse signal in the time domain that is spread monotonically in the frequency domain. A receiver converts the spread frequency signal back into an impulse signal. These frequency-spread impulse signals have applications in radar, for the pulse position modulation of information, or both, such as the R.sup.3 transponder developed by General Dynamics, Electronics Division in the 1970s. Frequency hopping systems communicate by synchronizing users to simultaneously change the communication frequency.
Direct Sequence systems spread a digital stream of information, typically in a quadriphase modulation format, with a PN code generator, to phase shift key modulate a carrier signal. The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code. Direct Sequence systems have excellent immunity to noise. The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user's PN codes. However, Direct Sequence system require large RF bandwidths and long acquisition times.
A third generation, wideband CDMA (W-CDMA) system is in development as described in "Wideband-CDMA Radio Control Techniques for Third Generation Mobile Communication Systems", written by Onoe et al., IEEE 47.sup.th Vehicular Technology Conference Proceedings, May 1997, that may have global applications. Instead of a pilot channel, the W-CDMA system has a broadcast, or perch channel. Each timeslot, or slot of the broadcast channel consists of a series of time multiplexed symbols. A long code masked, or special timing symbol segment uses just a short code to spread one symbol of known information. This segment allows a mobile station to acquire system timing information immediately after turn-on.
In most communication systems the signal is transmitted around a carrier signal which has a certain frequency. A receiver in the system acquisitioning this signal must ensure that its local oscillator frequency is operating a frequency very close to that of the transmitters to ensure demodulation. The control of the local oscillator is maintained through a mechanism called automatic frequency control (AFC).
Many cellular communication systems use a wideband signal for the transmission of data. The wideband carrier signal permits the receiver to resolve the transmitted signal into a number of paths. Each path can be used to generate a signal to control the frequency of the local oscillator. Typically, in cellular communications the wideband signal used is a CDMA signal.
Any communication system needs a close tracking of the frequency of the received carrier. Although CDMA systems are robust in the reception of multipathed signals, it is still necessary to properly acquire the carrier frequency to ensure proper demodulation. Receiver clock inaccuracies, transmitter frequency drift, and doppler effects require that the CDMA receiver track and adjust the carrier frequency of each multipath signal.
FIG. 1 illustrates the automatic frequency control (AFC) system 10 of a CDMA receiver (prior art). The received signal on lines 12a, 12b, and 12c is sampled at a frequency of f.sub.s samples/sec. Each of the signals is then matched to a local oscillator (LO) frequency generated by a Numerically Controlled Oscillator (NCO) 14, and then passed to frequency detectors 16a, 16b, and 16c. That is, each despread signal is mixed with the LO with mixers 18a, 18b, and 18c, and a down-converted signal, proportional to the frequency of its corresponding despread signal is generated and passed to frequency detectors 16a, 16b, and 16c. The outputs of frequency detectors 16a, 16b, and 16c are error signals proportional to the difference in the carrier frequency of the signal and the local oscillator frequency. These error signals are summed in combiner 19 before application to NCO 14. Frequency detection is achieved by matching the despread signal with a delayed version of its own and then performing the phase detection on it. Some systems shift the signal in the detection process. This shift is compensated for by pre-shifting the despread signal before introduction to frequency detectors 16a, 16b, and 16c.
Combining error signals increases the reliability of tracking he carrier frequency, as opposed to using the error signal from only one path. However, as can be seen from FIG. 1, several frequency detectors are required after pre-shifting each of the despread signals. This makes the tracking scheme very complex, requiring several multiplication operations.
Co-pending patent application, Ser. No. 09/015,424, invented by Kowalski et al. entitled SYSTEM AND METHOD FOR CDMA CHANNEL ESTIMATION, attorney docket no. SMT 301, filed on Jan. 29, 1998, and assigned to the same assignees as the instant application, discloses a procedure for using timing derived from the perch channel in a wideband CDMA system to despread and demodulate the traffic channels. Although the system simplifies the operation of the traffic channel, no particular system for simplifying the AFC is presented.
Co-pending patent application, Serial No. 09/048,240 invented by Kowalski et al. entitled PILOT AIDED, TIME-VARYING FINITE IMPULSE RESPONSE, ADAPTIVE CHANNEL MATCHING RECEIVING SYSTEM AND METHOD, attorney docket no. SMT 315, filed on Mar. 25, 1998, and assigned to the same assignees as the instant application, discloses a system which simplifies the timing and demodulation of traffic channels, but no specific system to simplify the AFC is disclosed.
It would be advantageous if a CDMA receiver design could simplify the AFC function to reduce the number of parts, decrease the receiver's power consumption, and reduce the number of arithmetic operations.
It would be advantageous if the AFC function in a CDMA receiver could be simplified without impacting the accuracy and performance of the receiver in tracking carrier frequencies.
It would be advantageous if the number of frequency discriminators, or frequency discriminator operations could be reduced without impacting the accuracy and performance of the receiver in tracking carrier frequencies.
Accordingly, in a wideband wireless communication system, such as a CDMA or W-CDMA system, a method for generating automatic lo frequency control (AFC) to track the carrier frequency of communication signals received along a plurality of transmission paths, with corresponding Doppler frequency shifts or frequency errors is provided. Typically, such a system includes mobile stations receiving communications from at least one base station. The method comprising the steps of:
a) combining carrier signals from each transmission path, whereby an average carrier signal is derived; and PA1 b) in response to the average carrier signal derived in Step a), calculating the frequency error of the average carrier signal. In this manner, only a single frequency error computation is performed. The frequency of the carrier signal received is not always the same frequency as transmitted. Therefore, even when the carrier frequency is known at the receiver, adjustment and acquisition are necessary for proper demodulation. PA1 c) in response to the frequency error calculated in Step b), changing the reference frequency, whereby the down-converted frequency of each transmission path is corrected with a single error signal. PA1 d) comparing the signal to noise ratio of the down-converted signal of each transmission path with the signal to noise ratio of the average down-converted signal generated in Step a); PA1 e) in response to the difference between the signal to noise ratios calculated in Step d), calculating a weight for each transmission path; and PA1 in which Step a) includes, in response to the weights calculated in Step d), variably adjusting the importance of each transmission path in the averaging of the down-converted signals, hereby the signal to noise ratio of the combination of received signals is improved. PA1 1) detecting changes between the current average down-converted signal and the average down-converted signal in storage, whereby a frequency change is measured; PA1 2) generating an error signal in response to the frequency change detected in Step b)2); and PA1 in which Step c), in response to the error signal generated in Step b)2), includes changing the reference frequency. PA1 sampling the received communication signals at a rate of f.sub.s ; PA1 measuring a time delay equal to 1/f.sub.s ; PA1 disregarding communication signals received after the 1/f.sub.s time delay has expired; and PA1 in which Step a) includes summing the received communication signals having a time delay less than, or equal to the time duration 1/f.sub.s, whereby the average received communication signal frequency error is less noisy.
The advantage of the present invention is that only one average need be calculated from the input signals of many transmission paths. As is shown below, the single average is used to correct all the input carrier signals. The inventions permits the accuracy derived from using several input carrier signals frequencies in the average calculation, with the simplicity of calculating a single average.
Typically, the carrier signal is down-converted for processing. Then, a reference signal, having a reference frequency, is generated. The reference signal is a local oscillator (LO) signal with a LO frequency. In response to a comparison of the reference signal and the carrier signal for each transmission path, a down-converted signal is generated for each transmission path. The step of generating each down-converted signal includes mixing the LO signal with the received carrier signal of each transmission path. Step a) then includes combing the down-converted signals.
The method further comprising a step, following Step b), of:
The method includes the further steps of:
In some aspects of the invention, the previous average down-converted signal is stored. Then, Step b) includes the sub-steps of:
Each transmission path has a corresponding time delay, and further steps, precede Step a), of:
An automatic frequency control (AFC) system to track the carrier frequency of received multipath communications is also provided comprising a combiner having a plurality of inputs, with each input corresponding to a transmission path. The combiner accepts received communication signals with received communication frequencies, and has an output to provide an average received communication signal. The AFC system also comprises a frequency error calculator having an input operatively connected to the combiner output to accept the average received communication signal. The frequency error calculator has an output to provide an frequency error signal in response to the average received communication signal, whereby the multipathed received signals are averaged before the frequency error is calculated.
A plurality of fingers parallely process received communication signals. Each finger has an input to accept a received communication signal corresponding to a transmission path and provides the received communication signal at an output operatively connected to a corresponding input of the combiner.
A multiplier has a first input to accept a reference signal with a reference frequency, and second input to accept the received carrier signals. The multiplier mixes the carrier signals with the reference signal to provide down-converted signals with received frequencies at an output. The fingers each accept a down-converted signal from the multiplier output, and the combiner provides an average down-converted signal.
The frequency error calculator includes a frequency discriminator having an input operatively connected to the combiner output to accept the average down-converted signal. The frequency discriminator measures the change in the average down-converted signal frequency and provides the error signal at an output.